A large enterprise network has two data centers and a WLAN edge with a large hub-and spoke network. The complete network is configured as a single OSPF area, and spoke routers are connected to…

Question

Options

• APlace spoke routers in stub areas
• BMake the hub routers ABR
• CMake the hub routers ASBR
• DPlace spoke routers in totally stubby areas
• EKeep the spoke routers in normal areas

Explanation

A (stub areas) and B (hub routers as ABRs) are the correct choices.

Placing spoke routers in stub areas (A) prevents Type 5 External LSAs from flooding across unreliable WAN links - spoke routers receive a single default route instead of the full LSDB, dramatically reducing OSPF overhead and churn caused by link instability. This requires splitting the current flat single-area design into multiple areas, which means the hub routers must become ABRs (B) - they sit at the boundary between the backbone area and the spoke stub areas, injecting that default route into each stub.

C is wrong because ASBRs redistribute external routes into OSPF - that increases LSA flooding rather than reducing it. D (totally stubby) is a Cisco-proprietary extension that also blocks Type 3 Summary LSAs; while valid in practice, stub areas (A) are the standard OSPF answer here and are sufficient. E is wrong because keeping normal areas is the current broken design - normal areas receive all LSA types, which is exactly the problem on unreliable WAN links.

Memory tip: Think "hub = border, spoke = stub." Hub routers border multiple areas (ABR), and spoke routers get stubbed out from full LSA flooding - they only need a default route to reach the rest of the world.

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